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If simple mass absorbed sound, SONAR would be useless. If mass absorbed sound, steel I-beams in buildings wouldn't ring like a frellin' bell and transmit sound all over the structure. If mass absorbed sound, stone cathedrals would be silent.

Certain types of mass are efficient at converting vibration into heat so that it can dissipate. Many are better at transmitting the sound than they are at dissipating it.

The reason that the wedge and pyramid shaped sound deadening surfaces inside an anechoic chamber works isn't because it's massive (it's not at all massive), it's because of its design.

The reason it won't die is because it is a good rule of thumb to give someone without dropping a 1000 page book about acoustics on them. It is true that it must be the right mass to be effective but a bookcase isn't a steel I-beam nor a stone cathedral (in which there's a lot more at play than mass). I don't think anyone would use those items for this purpose and an apartment is not an anechoic chamber. The point of the 'myth' is to illustrate that a blanket is going to be better than a sheet. By pointing out that a lot of mass is needed to block bass I was merely illustrating that you can't (without a foot or more of concrete).

Maybe I wasn't clear enough (I do tend to ramble) in pointing out I was advocating diffusion as the primary and best method of cleaning up a sound space, making the space sound nice rather than isolated. For music my second port of call is roof insulation (I believe its called glass wool in the US). You can get it in rolls with a foil backing which works great because the fibre glass strands are effective as diffusion and the foil takes the energy out (it takes more energy to vibrate the foil than the vibrations give back). I didn't mention it because unless its a permanent space its probably more effort than you want to go to.

In the real world you rarely have what you want and knowing a few tricks to make do with what you have will get you a long way. I've never recorded in a high end studio or even a low end one for that matter. I'm usually in someone's lounge, garage or home studio with mattresses and my rubber underlay used stuffed into corners or erected into baffles, cleaning up the acoustics as best I can.

Internal acoustics is treating the space. External noise abatement is keeping the neighbor's dog from being heard while you record or keeping your guitar amp sound out of their space.

For most noise abatement issues, although the best block does involve mass, simple mass is not where near effective as D-I-D; Density-Isolation-Density. Two masses that are isolated by a space so they don't connect. That's usually used to keep the noises from one space from getting into the other. Putting sound blankets on your wall won't do much for the dog and guitar amp scenario because there's only one mass; the wall.

For Internal acoustic issues, you need a balance of diffusion and absorption. The bookcase idea mentioned earlier is a very good example of diffusion. Blankets and foam panels are absorption. Fiberglass is not used a lot anymore because small particles of it will work their way free and can be inhaled. Blankets and foam panels are not as effective at low frequencies. Bass traps need to be designed and built. If you're on location and low frequencies really are a problem, you need to find a better location because there isn't enough money to do the job.

Think of a space like a coke bottle. When you blow over the top, you provide enough energy to excite the bottle to its resonant frequency and it whistles. Some spaces are very resonant; some aren't. Spaces can be resonant at one or any number of frequencies. And there's also the issue of where the sound is generated from within the space. If your talent is loud enough, he/she can excite the space with his/her voice and all sorts of things will be heard.

As mentioned earlier, spanning sound blankets across the ceiling works as does covering the floor and building a sound block of blankets just out of frame. That's a lot of blankets, but usually far less than it would take to blanket the entire room. The idea is to stop reflections from the big, hard, flat spaces, and also to reduce the energy of the voice to keep it from hitting the walls, ceiling and floor. Building a sound block of blankets just out of frame achieves this, by blocking and absorbing some of the energy BEFORE it makes the room ring.